That's true. HOWEVER, I'm pretty sure I can safely say that they would likely have sold at least as well if they were last gen only games, and the last gen ports from Watch Dogs, Titanfall, The Evil Within, and Destiny prove that they didn't necessarily need next gen for them to work.
Most last gen games in 2013 flopped pretty hard. New IPs would probably have a hard, hard time. And all of these cross gen titles ended up selling way more on PS4, even with a smaller base. This shows that the active base it's on next gen and things will only increase. And if the early adopter moved to next gen, launching a new franchise on last gen would probably only gather a following of late adopters. So when PS360 winds down, these games would end up not having a decent footprint on next gen until the late adopters jumped (end of gen).
Despite that, PS3 and 360 won't result in good sales after 2015. PS4 and X1 will give 3rd parties good numbers until 2019/20. The transition between gens is the right time to estabilish a new franchise. And you have to think in the future, and that's PS4 and X1.
We can think that these franchises could have been succesfull on last gen only. But a lot of people already moved to next gen and aren't buying a lot of games for the old consoles or simply don't own a last gen console anymore. So let's think in the following scenario: let's suppose that CoD Ghosts and MW were PS360 only. People on PS4/X1 would probably prefer their new machines with better graphics to play or simply they sold their old console or it broke and they won't bother with replacing it. Maybe they decide to buy the alternative, that could be BF4/Hardline. That could lead to a situation where BF became the biggest shooter franchise on next gen and as the sales on PS360 fall, the sales on PS4/X1 would rise. In the end, the last gen franchise would end up damaged. These is clear in our real world scenario: both CoD and BF immediatily set their feet on PS4/X1 at launch. Other franchises did the same, like NFS.